1. Field of the Invention
The present invention relates to a fixing apparatus incorporated in an image forming apparatus such as a copying machine or printer using electrophotography and, more particularly, to a heat fixing apparatus which heats a recording medium bearing an unfixed image formed on it, thereby fixing the image.
2. Description of the Related Art
Image forming apparatuses such as a copying machine or printer using electrophotography widely use a heat fixing apparatus which heats a recording medium bearing an unfixed image formed on it, thereby fixing the image. Generally, such a heat fixing apparatus often includes a heating element serving as a heat source, a power supply that supplies a current to the heating element, a temperature detection means for detecting the temperature near the heating element, and a control means for controlling the current to be supplied to the heating element. If even one of the heating element, power supply, temperature detection means, and control means fails to normally function, the fixing apparatus cannot normally operate. For example, if energization runaway has occurred, the apparatus may suffer damage due to overheat. In general, a safety element device provided in the fixing apparatus suppresses overheat in the event of energization runaway.
Various techniques have been developed against this problem. As a safety device for de-energizing a heating element, Japanese Patent Laid-Open No. 08-248813 discloses a safety element such as a thermal switch or fuse which is actuated upon detecting an abnormal temperature rise by itself, independently of a system control unit.
However, the actuation of the safety element sometimes delays. For example, the actuation delays if large power is supplied to the heating element in a rotation stop state of a pressing member. In this case, since the amount of heat dissipated from the heating element via the pressing roller decreases, it is high probable that the temperature of the heating element abruptly rises. When the temperature of the heating element abruptly increases, the safety element such as a thermal switch cannot quickly follow the temperature rise. This may cause thermal damage to the apparatus before the safety element is actuated.
Japanese Patent Laid-Open No. 2004-102121 discloses a fixing apparatus which has a rotation detection sensor for detecting the rotation of a heating roller and, in accordance with the rotation state of a control heating roller that controls energization to a coil for making the heating roller generate heat, limits the energization amount to the coil.
When the temperature of the heating roller readily rises abruptly, i.e., when the heating roller stops rotation, the control unit receives a detection result from the rotation detection sensor and stops or decreases the energization amount to the coil.
In the arrangement disclosed in Japanese Patent Laid-Open No. 2004-102121, however, if the control unit fails, the energization amount to the coil cannot be limited. As a result, even when the rotation detection sensor detects the rotation state of the heating roller, the apparatus may suffer thermal damage before the safety element such as a thermal switch is actuated.